The Compagnie of Uffz- Garde du Corps at the Palace was never 1621. In 1871 it had 1657, and in 1897, as the creation date was incorrect, the 1657 was replaced by 1623. The helmet was made of nickel silver after 1871. (The 115 only in 1897).
Here is a page documenting the banners.
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The helmets of the Garde-Schloss NCO company are extremely rare. No model prior to M1897 (nickel silver lion with double crown of foliage) has ever been seen. I have only seen one helmet in 50 years, and 2 are known at most. Here's one of the 2, proposed by ‘Sergio’ a few years ago.
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It would appear that all these additive banners were decided on by the Grand Duke when he was placed under Prussian nomenclature in 1871.